In plasma coating technology, coatings are applied by injecting a powder into a plasma stream and impinging the stream upon a surface of an object to be coated. In order to coat the entire surface, the point at which the stream impinges upon the surface or the impingement point, is moved over the surface, by moving the torch body and/or the object. In order to achieve an optimal coating, the angle of the axis of the plasma stream to the substrate surface at the moving impingement point should be maintained perpendicular. In addition, the standoff, the distance between the nozzle of the torch and the impingement point should be maintained substantially constant as the impingement point moves over the surface. An additional requirement for an optimal coating is that the impingement point move across the surface at a substantially constant rate.
For simple surfaces of revolution, such as cylinders, conical sections, and annular surfaces, these requirements are met by merely rotating the object and moving the torch at a constant rate along a straight line.
For more complex shapes, achieving these requirements is more difficult. To coat, for example, a surface of rotation, such as a spherical surface, the torch must be moved along a curved path to maintain a constant standoff, and be rotated to maintain a perpendicular impingement angle. The constant standoff, and perpendicular impingement angle can be provided by an apparatus that moves the torch in the direction parallel and the direction perpendicular to the axis of rotation of the object, and rotates the torch on an axis perpendicular to the plane defined by these directions. However, under certain circumstances, the rotation of the torch introduces variation in the rate of movement of the impingement point over the surface. For example, when the axis of rotation is through the torch body, rotation of the torch through an angle sweeps the plasma spray across the substrate at a varying speed. Unless the sweeping motion is compensated by complex motion programming of the torch, the resulting variation in speed leads to a suboptimal coating. It is possible to avoid the sweeping motion altogether by rotating the torch about the axis passing through the impingement point. Rotation about this axis generally requires movement of a bulky support and supply structure for the torch. Due to space limitations, it is often difficult or impossible to not only provide this movement, but to additionally provide for a constant standoff and perpendicular spray axis.
For tubular objects with irregular internal surfaces the requirements for achieving an optimal coating are particularly difficult to achieve. Such objects include tubular objects with essentially an internal surface of rotation with an irregular profile in the direction of the longitudinal axis, or tubular objects with an internal surface of constant profile in the direction of a longitudinal axis, but with non-circular cross-sections perpendicular to a longitudinal axis. When the internal surface of the tubular object is irregular, it is difficult or impossible to achieve these motions with a conventional apparatus. The difficulty is due in large part to the support and supply apparatus required for a plasma torch. A plasma torch is typically connected to one or more gas supply lines, a powder supply line, two large electrical cables, and two cooling water hoses. These are bound together in a bundle as a rather inflexible service connection. The service connection and the torch itself are attached to a rigid arm which is in turn attached to the mechanism for moving to torch through the desired motions. As the torch moves over the internal surface, the rigid arm, the torch, and/or the service connection generally run into the object being coated, thus preventing the desired motions of the torch. This problem is particularly severe as the length to diameter ratio of the object increases.
Confronted with the difficulties of moving the torch in a manner to achieve the optimal coating quality on internal surfaces of tubular objects, the practitioner must either avoid the coating of certain complex shapes, or be satisfied with a coating of suboptimal quality.